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                        <h1 id="the-design-philosophy-of-the-darpa-internet-protocols"><a class="header" href="#the-design-philosophy-of-the-darpa-internet-protocols">The Design Philosophy of The DARPA Internet Protocols</a></h1>
<h1 id="abstract"><a class="header" href="#abstract">Abstract</a></h1>
<p>The Internet protocol suite, TCP/IP, was first proposed fifteen years ago. It was developed by the Defense Advanced Research Projects Agency (DARPA), and has been used widely in military and commercial systems. While there have been papers and specifications that describe how the protocols work, it is sometimes difficult to deduce from these why the protocol is as it is. For example, the Internet protocol is based on a connectionless or datagram mode of service. The motivation for this has been greatly misunderstood. This paper attempts to capture some of the early reasoning which shaped the Internet protocols.</p>
<h1 id="introduction"><a class="header" href="#introduction">Introduction</a></h1>
<p>For the last 15 years [1], the Advanced Research Projects Agency of the U.S. Department of Defense has been developing a suite of protocols for packet switched networking. These protocols, which include the Internet Protocol (IP), and the Transmission Control Protocol (TCP), are now U.S. Department of Defense standards for internetworking, and are in wide use in the commercial networking environment. The ideas developed in this effort have also influenced other protocol suites, most importantly the connectionless configuration of the IS0 protocols [2,3,4].</p>
<p>While specific information on the DOD protocols is fairly generally available [5,6,7], it is sometimes difficult to determine the motivation and reasoning which led to the design.</p>
<p>In fact, the design philosophy has evolved considerably from the first proposal to the current standards. For example, the idea of the datagram, or connectionless service, does not receive particular emphasis in the first paper, but has come to be the defining characteristic of the protocol. Another example is the layering of the architecture into the IP and TCP layers. This seems basic to the design, but was also not a part of the original proposal. These changes in the Internet design arose through the repeated pattern of implementation and testing that occurred before the standards were set.</p>
<p>The Internet architecture is still evolving. Sometimes a new extension challenges one of the design principles, but in any case an understanding of the history of the design provides a necessary context for current design extensions. The connectionless configuration of IS0 protocols has also been colored by the history of the Internet suite, so an understanding of the Internet design philosophy may be helpful to those working with ISO.</p>
<p>This paper catalogs one view of the original objectives of the Internet architecture, and discusses the relation between these goals and the important features of the protocols.</p>
<h1 id="fundamental-goal"><a class="header" href="#fundamental-goal">Fundamental Goal</a></h1>
<p>The top level goal for the DARPA Internet Architecture was to develop an effective technique for multiplexed utilization of existing interconnected networks. Some elaboration is appropriate to make clear the meaning of that goal.</p>
<p>The components of the Internet were networks, which were to be interconnected to provide some larger service. The original goal was to connect together the original ARPANET[8] with the ARPA packet radio network[9,10], in order to give users on the packet radio network access to the large service machines on the ARPANET. At
the time it was assumed that there would be other sorts of networks to interconnect, although the local area network had not yet emerged.</p>
<p>An alternative to interconnecting existing networks would have been to design a unified system which incorporated a variety of different transmission media, a multi-media network.</p>
<blockquote>
<p>Perhaps “multi-media” was not well-defined in 1988. It now has a different meaning, of course.</p>
</blockquote>
<p>While this might have permitted a higher degree of integration, and thus better performance, it was felt that it was necessary to incorporate the then existing network architectures if Internet was to be useful in a practical sense. Further, networks represent administrative boundaries of control, and it was an ambition of this project to come to grips with the problem of integrating a number of separately administrated entities into a common utility.</p>
<p>The technique selected for multiplexing was packet switching.</p>
<p>An alternative such as circuit switching could have been considered, but the applications being supported, such as remote login, were naturally served by the packet switching paradigm, and the networks which were to be integrated together in this project were packet switching networks. So packet switching was accepted as a fundamental component of the Internet architecture. The final aspect of this
fundamental goal was the assumption of the particular technique for interconnecting these networks. Since the technique of store and forward packet switching, as demonstrated in the previous DARPA project, the ARPANET, was well understood, the top level assumption was that networks would be interconnected by a layer of Internet packet switches, which were called gateways.</p>
<p>From these assumptions comes the fundamental structure of the Internet: a packet switched communications facility in which a number of distinguishable networks are connected together using packet communications processors called gateways which implement a store and forward packet forwarding algorithm.</p>

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